Debt (Narratives) (Details) (USD $)	6 Months Ended					0 Months Ended				0 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013	Jun. 30, 2014 Notes And Mortgages Payable Fixed Rate [Member]	Dec. 31, 2013 Notes And Mortgages Payable Fixed Rate [Member]	Apr. 16, 2014 Notes And Mortgages Payable Fixed Rate [Member] New York 909 Third Avenue [Member] sqft	Mar. 31, 2014 Notes And Mortgages Payable Fixed Rate [Member] New York 909 Third Avenue [Member]	Jun. 30, 2014 Notes And Mortgages Payable Variable Rate [Member]	Dec. 31, 2013 Notes And Mortgages Payable Variable Rate [Member]	Jun. 30, 2014 Notes And Mortgages Payable Variable Rate [Member] New York Manhattan 220 Central Park South Site [Member]	Jan. 31, 2014 Notes And Mortgages Payable Variable Rate [Member] New York Manhattan 220 Central Park South Site [Member] Extensions	Jun. 16, 2014 Senior unsecured notes Due June 30, 2019 [Member]
Debt Instrument [Line Items]
Mortgages payable (in US Dollars)	$ 8,988,843,000		$ 8,331,993,000	$ 7,623,049,000	$ 7,563,133,000		$ 193,000,000	$ 1,365,794,000	$ 768,860,000		$ 600,000,000
Notes And Loans Payable Refinanced Amount (in US Dollars)						350,000,000
Debt Term						7 years
Interest Rate, End of Period (in percentage)				4.48%		3.91%	5.64%	2.31%
Proceeds from borrowings (in US Dollars)	1,398,285,000	1,583,357,000				145,000,000
Square Footage Of Real Estate Property						1,300,000
Length Of Extension Available											1 year
Number Of Extensions Available											3
Debt Instrument Maturity						May 2021					January 2016
Spread Over LIBOR (in percentage)										2.90%	2.75%
Public Offering Of Notes Payable (in US Dollars)												$ 450,000,000
Public Offering, Interest Rate												2.50%
Debt Instrument, Issuance Price, Percentage												99.619%
Interest Rate, Effective												2.581%